Make Darwin HostNameRegistrar safer to use.

[bzbarsky-apple]

There are two main changes here:

1. When we shut down the Matter stack, we should remove all our advertisements adn stop monitoring for network interface changes. That's what the change to ChipDnssdShutdown does.

2. It's theoretically possible for the block HostNameRegistrar passes to nw_path_monitor_set_update_handler to run after the HostNameRegistrar is destroyed. Guard against that with a shared_ptr that keeps track of the liveness state of the HostNameRegistrar.

The changes to the member initialization of HostNameRegistrar are just for safety's sake, in case its destructor ever gets called without Init() having been called first. Right now that is not possible, but better to not rely on that.

[github-actions]

PR #25983: Size comparison from 7c2e7db to 5058593

Decreases (1 build for cc32xx)

platform	target	config	section	7c2e7db	5058593	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	20302987	20302986	-1	-0.0

Full report (6 builds for cc32xx, linux, mbed, qpg)

platform	target	config	section	7c2e7db	5058593	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	642745	642745	0	0.0
			(read/write)	203848	203848	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197248	197248	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	933099	933099	0	0.0
			.debug_aranges	87608	87608	0	0.0
			.debug_frame	301328	301328	0	0.0
			.debug_info	20302987	20302986	-1	-0.0
			.debug_line	2679748	2679748	0	0.0
			.debug_loc	2824679	2824679	0	0.0
			.debug_ranges	286200	286200	0	0.0
			.debug_str	3039406	3039406	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	104289	104289	0	0.0
			.shstrtab	232	232	0	0.0
			.stab	204	204	0	0.0
			.stabstr	441	441	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	377422	377422	0	0.0
			.symtab	256768	256768	0	0.0
			.text	536336	536336	0	0.0
linux	chip-tool-ipv6only	arm64	(read only)	12113692	12113692	0	0.0
			(read/write)	742616	742616	0	0.0
			.bss	34392	34392	0	0.0
			.data	3008	3008	0	0.0
			.data.rel.ro	684496	684496	0	0.0
			.dynamic	560	560	0	0.0
			.got	15504	15504	0	0.0
			.init	24	24	0	0.0
			.init_array	216	216	0	0.0
			.rodata	566156	566156	0	0.0
			.text	9785268	9785268	0	0.0
	thermostat-no-ble	arm64	(read only)	2500492	2500492	0	0.0
			(read/write)	145192	145192	0	0.0
			.bss	56344	56344	0	0.0
			.data	1784	1784	0	0.0
			.data.rel.ro	77648	77648	0	0.0
			.dynamic	560	560	0	0.0
			.got	5360	5360	0	0.0
			.init	24	24	0	0.0
			.init_array	432	432	0	0.0
			.rodata	129400	129400	0	0.0
			.text	2108784	2108784	0	0.0
mbed	lock-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2465472	2465472	0	0.0
			.bss	215964	215964	0	0.0
			.data	5880	5880	0	0.0
			.text	1428116	1428116	0	0.0
qpg	lighting-app	qpg6105+debug	(read/write)	1153440	1153440	0	0.0
			.bss	97404	97404	0	0.0
			.data	852	852	0	0.0
			.text	600536	600536	0	0.0
	lock-app	qpg6105+debug	(read/write)	1122048	1122048	0	0.0
			.bss	92540	92540	0	0.0
			.data	856	856	0	0.0
			.text	569148	569148	0	0.0

[bzbarsky-apple]

Fast-tracking platform-specific fix with platform owner review.